AISI 4340 is a high-strength nickel–chromium–molybdenum (Ni–Cr–Mo) hardenable low-alloy steel engineered for applications demanding exceptional toughness, very high fatigue strength, and deep hardenability.
Unlike medium-strength Cr–Mo steels such as 4140, the nickel addition in 4340 significantly enhances core toughness, low-temperature performance, and resistance to shock loading, even at very high strength levels.
Chemical Composition
| Element | Content (%) | Role |
|---|---|---|
| Carbon (C) | 0.38 – 0.43 | Strength & hardness |
| Nickel (Ni) | 1.65 – 2.00 | Toughness, fatigue resistance |
| Chromium (Cr) | 0.70 – 0.90 | Hardenability, wear resistance |
| Molybdenum (Mo) | 0.20 – 0.30 | High-temperature strength |
| Manganese (Mn) | 0.60 – 0.80 | Toughness |
| Silicon (Si) | 0.15 – 0.30 | Strength, deoxidation |
| Phosphorus (P) | ≤ 0.035 | Controlled impurity |
| Sulfur (S) | ≤ 0.040 | Machinability |
| Iron (Fe) | Balance | Base metal |
Metallurgical Structure
• Annealed: Ferrite + pearlite
• Quenched: Fully martensitic structure
• Tempered: Tempered martensite with high toughness
• Nickel improves grain refinement, ductility, and impact resistance
Mechanical Properties
| Condition | Tensile Strength | Yield Strength | Elongation | Hardness |
|---|---|---|---|---|
| Annealed | ~745 MPa | ~470 MPa | 22–25% | ~200 HB |
| Normalized | ~900 MPa | ~720 MPa | 18–20% | ~240 HB |
| Quenched & Tempered | 1,100 – 1,450 MPa | 950 – 1,250 MPa | 12–18% | 32–45 HRC |
Key Characteristics
Exceptional Hardenability
• Uniform hardness through very thick sections (>150 mm)
• Superior to 4140 in deep-section performance
Outstanding Toughness & Fatigue Resistance
• Nickel enhances impact toughness and fatigue life
• Maintains toughness at very high strength levels
High Wear & Abrasion Resistance
• High hardness achievable after heat treatment
• Surface treatments further enhance wear life
High Strength-to-Weight Ratio
• Enables weight reduction without performance loss
• Preferred for aerospace and defense structures
Heat Treatment & Refining
Annealing: 830–860°C – Improves machinability
Normalizing: 870–900°C – Grain refinement
Quenching: Oil or polymer – Fully martensitic structure
Tempering: 200–650°C – Balances strength and toughness
Surface Hardening: Nitriding, carburizing, induction hardening
Fabrication & Welding
• Good machinability in annealed or normalized condition
• Welding requires preheating (250–350°C) and post-weld stress relief
• Moderate formability; hot forming preferred
Available Forms
✔ Hot rolled & forged bars
✔ Shafts & discs
✔ Plates & flats
✔ Seamless mechanical tubes
✔ Billets, rings & heavy forgings
Standards & Equivalents
| Standard | Equivalent |
|---|---|
| AISI / SAE | 4340 |
| ASTM | A29 / A322 |
| EN | 34CrNiMo6 |
| DIN | 1.6582 |
| JIS | SNCM439 |
| BS | EN24 |
Applications
Aerospace & Defense: Landing gear, structural fittings, fasteners
Automotive & Motorsport: Crankshafts, gear shafts, driveline parts
Oil & Gas: Drill collars, tool joints, pressure components
Heavy Engineering: Turbine shafts, large gears, mining equipment
Advantages
✔ Exceptional toughness at high strength
✔ Deep hardenability for thick sections
✔ Outstanding fatigue resistance
✔ Excellent response to heat treatment
✔ Widely accepted aerospace-grade alloy
Limitations
⚠ Higher cost than 4140
⚠ Requires controlled welding procedures
⚠ Corrosion resistance lower than stainless steels